1. Field of the Invention
The present invention relates generally to a method of producing a dielectric ceramic composition which can be fired at a relatively low temperature, and more particularly to a method of producing such a low temperature firing dielectric ceramic composition for microwave applications, which is suitably used for a dielectric resonator having internal conductive layers, of a stripline type filter, for example. The present invention is also concerned with a dielectric resonator obtained by using such a dielectric ceramic composition, a dielectric filter including a plurality of such dielectric resonators, and with a method of producing the dielectric resonator or dielectric filter.
2. Discussion of the Prior Art
In a modern microwave telecommunication system such as a portable or automobile telephone system, there is widely used a coaxial type dielectric filter using a ceramic composition having a high dielectric constant. The coaxial type dielectric filter has a plurality of coaxial type resonators connected to each other. Each resonator is a cylindrical dielectric block which has inside and outside conductors formed on inner and outer circumferential surfaces of the block, respectively. This type of dielectric filter has a limitation in reducing the size and thickness thereof due to its construction. In view of this, there is proposed a stripline type filter of a tri-plate structure, which incorporates internal conductive layers or strips within a dielectric substrate. In this stripline type filter, a patterned array of conductors in the form of strips are integrally embedded in the dielectric substrate so as to provide a plurality of resonators. The thus constructed stripline type filter is comparatively compact and thin.
In fabricating such a stripline type dielectric filter having the internal conductive layers or strips as described above, a dielectric ceramic composition which gives the dielectric substrate must be co-fired with the internal conductive layers. Since known dielectric ceramic compositions have a considerably high firing temperature, there is a limit to conductive materials which can be used for the internal conductive layers, thus making it difficult to employ an Ag-containing material having a relatively low conductivity resistance. In microwave circuits incorporating conductors therein, especially in stripline type filter devices, Ag or Cu having low conductivity resistance needs to be used so as to reduce signal loss. Cu conductors must be fired in a non-oxidizing atmosphere, such as nitrogen, since Cu forms oxides when fired in an oxidizing atmosphere. However, the firing under the non-oxiding atmosphere makes it difficult to remove an organic binder which is added to an unfired ceramic body, and also causes problems such as relatively high processing cost. While Ag conductors may be fired in an oxiding atmosphere, Ag has a relatively low melting point of 962.degree. C., and thereby requires the use of a dielectric ceramic composition for cofiring therewith, which can be fired at 962.degree. C. or lower, desirably at 950.degree. C. or lower, more desirably 900.degree. C. or lower.
On the other hand, the dielectric ceramics used in the microwave applications need to have a high specific dielectric constant, which leads to reduction in the length of resonators used in the filter devices described above, making the devices small-sized. When the devices are used at a relatively low microwave frequency around 1 GHZ, the length of the resonators tends to be increased due to a relatively long wavelength. In this case, the specific dielectric constant must be increased so as to shorten the length of the resonators. Further, the unloaded Q of the dielectric ceramics, which affects the characteristics of the devices, such as insertion loss of filters, must be kept at a high value in the microwave region. While the length of resonators tends to be reduced at higher frequencies, due to an accordingly short wavelength, the dielectric ceramics used desirably has a sufficiently large unloaded Q since the Q value tends to be reduced at higher frequencies. (f.Q=const. f=frequency).
Among various dielectric ceramic compositions which have been proposed, a dielectric ceramic composition which contains oxides of Ba, Ti, RE (rare earth metals) and Bi is known as having a high specific dielectric constant, a large unloaded Q, and a small temperature coefficient of the resonance frequency. Also are known dielectric ceramics having as a major crystal phase a solid solution in the BaO--RE.sub.2 O.sub.3 --4TiO.sub.2 or BaO--RE.sub.2 O.sub.3 --5TiO.sub.2 system. Although those dielectric ceramics are known as having a relatively high specific dielectric constant, they have a disadvantage in firing temperature which is as high as 1250.degree. C. or more. In view of this, various attempts have been made to lower the firing temperature, by addition of oxides of Pb, for example.
An example of such dielectric ceramic composition is disclosed in U.S. Pat. No. 3,811,937, wherein a calcined powder of BaO, TiO.sub.2 and a rare earth oxide is blended with 8 to 30% by weight of a glass formulation containing CdO, PbO and Bi.sub.2 O.sub.3. The thus prepared composition is fired at a temperature between about 982.degree. C. and 1150.degree. C. Another example of dielectric ceramic composition as disclosed in JP-A-59-214105 contains BaO, TiO.sub.2 and Nd.sub.2 O.sub.3 as major components, which are mixed with powders of PbO, Bi.sub.2 O.sub.3, SiO.sub.2 and ZnO. This composition is fired at a temperature between 1050.degree. C. and 1150.degree. C. A further example of composition as disclosed in JP-B2-4-16884 contains BaTiO.sub.3, Nd.sub.2 O.sub.3, TiO.sub.2 and Bi.sub.2 O.sub.3 as major components, to which Pb.sub.3 O.sub.4, B.sub.2 O.sub.3, SiO.sub.2 and ZnO are added in respective suitable amounts. This composition is fired at a temperature between 1000.degree. C. and 1050.degree. C. A still further example of dielectric ceramic composition as disclosed in JP-A-2-44609 contains BaTiO.sub.3, Nd.sub.2 O.sub.3, TiO.sub.2, Bi.sub.2 O.sub.3 and Pb.sub.3 O.sub.4 as major components, to which 2CaO.3B.sub.2 O.sub.3, SiO.sub.2 and ZnO are added. This composition is fired at a temperature between 1000.degree. C. and 1050.degree. C.
The known dielectric ceramic compositions as described above, which can be fired at a relatively low temperature, still has a firing temperature of around 1000.degree. C. or higher, and thus cannot be used with internal conductors formed solely of Ag having a low conductivity resistance, or alloys consisting principally of Ag. In fact, these compositions can be used only with internal conductors formed of Ag--Pd alloys including a relatively high content of Pd having a large conductivity resistance.
While some known techniques are available for lowering the firing temperature of a dielectric ceramic composition down to around 1000.degree. C., there have been unknown such techniques as permitting the firing at a temperature lower than the melting point of Ag, i.e., 962.degree. C., desirably at 950.degree. C. or lower, more desirably at around 900.degree. C.